This invention relates to a method of treating an aluminosilicate zeolite to remove tetrahedral aluminum therefrom.
A vast number of naturally-occurring and synthetic aluminosilicate zeolites are known. Among the more commercially important zeolites are zeolite Y (U.S. Pat. No. 3,130,007), zeolite beta (U.S. Pat. No. 3,338,069, reissued as RE. No. 28341) and ZSM-5 (U.S. Pat. No. 3,702,886). These zeolites find application in a wide variety of commercial processes, particularly hydrocarbon conversion processes.
The silica-to-alumina ratio of a zeolite is often variable; for example, zeolite Y can be synthesized with a silica-to-alumina ratio ranging from 3 to about 6. In some zeolites, for example ZSM-5, the upper limit of silica-to-alumina ratio is virtually unbounded. Thus U.S. Pat. No. 3,941,871 discloses a crystalline zeolite essentially free of aluminum and exhibiting an X-ray diffraction pattern characteristic of ZSM-5. Similarly, U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe microporous crystalline silicates or organosilicates in which the aluminum content is at impurity levels.
In addition, it is possible to vary the silica-to-alumina ratio of a naturally-occurring or already-synthesized zeolite by chemical treatment. In particular, in view of the relative instability of aluminum in the crystal lattice of a zeolite, a number of methods have been proposed for removing framework aluminum from a zeolite.
In U.S. Pat. No. 4,503,023 a method is disclosed for replacing framework aluminum in an aluminosilicate zeolite having a silica-to-alumina ratio of at least 3 zeolite with framework silicon by contacting the zeolite with an aqueous solution of a fluorosilicate salt having a pH of 3 to 7 and in an amount such as to provide at least 0.075 moles of fluorosilicate salt per 100 grams of the zeolite. However, although this method has proved effective in raising the silica-to-alumina ratio of zeolites, such as zeolite Y, the treated zeolite is found to contain impurities of insoluble metal fluorides, particularly AlF.sub.3, which are not readily removed from the zeolite by simple washing and which can adversely effect the catalytic properties of the zeolite.
In an attempt to overcome this problem, U.S. Pat. No. 4,597,956 discloses a method of treating a zeolite which contains insoluble fluoride with an aqueous solution of a soluble aluminum compound, such as aluminum sulfate, nitrate, chloride and acetate such that the mole ratio of fluoride anion to aluminum cation is reduced to a value less than 3, preferably 2-2.5.